It has been known that there are numerous examples of footwear mechanism actuated by means of heel-based pressure, applied when the user steps down. The most common being a weight-based pump or any pumps stowed inside the heel/sole in general of ventilating footwear. The sole or the heel must be thickened to give room for the pump. This affects the appearance of the footwear. The size, volume and sometimes, the shape of the pump are substantially limited by the shape, size and volume of the heel/sole. Also, this type of footwear is known to be uncomfortable and affects the stability of the footwear because of the vertical heel movement required to actuate the mechanism.
Thus in the few commercialized designs, the pump has been made to be quite small embedded within the sole thus limiting the negative impacts on stability but also adversely affecting the ventilating abilities of the pump as well.
Ventilating footwear with pumps actuated by substantially linear drive mechanism is available. However, the limitation on the shape, size and volume of the pump remains substantial as long as the pump is stowed inside the sole.